During the manufacturing of semiconductor devices, thin films are often formed on a wafer surface. A typical processing apparatus used to heat the wafers during this deposition or growth process is illustrated in FIG. 1. The processing apparatus used is often constructed so that a wafer 3 and a susceptor 5 may be heated from both sides by infrared radiation which is usually supplied by infrared lamps. The power to the heat sources, typically infrared lamps 1 and 7 may be adjusted so that the heat on the wafer side of the susceptor (typically the top side) is controlled separately from the heat on the opposing side of the susceptor (typically the bottom side) as illustrated in FIG. 1.
In the typical process apparatus, as shown in FIG. 1, power may be applied equally from the top and the bottom to heat the wafer 3 and the susceptor 5, respectively. Because the wafer 3 is thinner and has less thermal mass than the susceptor 5, the wafer 3 heats up at a much faster rate than the susceptor 5. This causes non-uniform heating of the wafer 3 and susceptor 5. In some cases, the wafer 3 can exceed the susceptor's temperature by more than 200.degree. C. during a heat up which could damage the wafer 3.
One solution to overcome this problem is to control the power to the top and bottom heat sources separately. As shown in FIG. 1, one pyrometer 9 is used to detect the temperature of the wafer 3 and a second pyrometer 11 is used to detect the temperature of the susceptor 5, as illustrated in FIG. 1. A pyrometer is a device that is sensitive to infrared radiation. The pyrometer collects light emitted from a substrate and then determines the temperature of the substrate based on the light emitted. The power of the top and bottom heat sources is controlled separately by controllers 15 and 13, respectively. The temperature of the susceptor 5 is ramped to a process temperature by controlling the power to the second heat source 7 using conventional Proportional Integral Derivative (PID) algorithm temperature ramping which is well-known to those skilled in the art. The pyrometer 11 monitors the susceptor's 5 temperature which is used to control the amount of radiant energy flux applied by the second heat source 7 to the back surface of the susceptor 5.
A second pyrometer 9 monitoring the wafer's temperature is used to control the radiant energy flux to the wafer side of the susceptor 5. A processing control system 17, such as a microprocessor and memory, compares the wafer temperature to the susceptor temperature and then adjusts the power of the wafer's heat source 1 in an attempt to keep the wafer 3 at approximately the same temperature as the susceptor 5 during the temperature ramp. The temperature of the wafer 3 may be set to be slightly ahead or slightly behind the temperature of the susceptor during the ramp.
By controlling the top and bottom heat sources separately, the temperature difference between the wafer and susceptor may be more accurately controlled.
Over time, the diameter of the wafers being processed has steadily increased. In addition, there is a trend toward further miniaturization of electronic circuits formed on these wafers. Of particular concern is the uniformity of wafer temperature to ensure uniform results, e.g., deposition thickness, across the diameter of the wafer during processing. Although the prior art method of controlling the temperature of the wafer and susceptor permit the temperature difference between the wafer and susceptor to be more accurately controlled, it does not provide a means for controlling the radial temperature gradients between the central and outer portions of the wafer and susceptor.
Thus, what is needed is a method and apparatus which can control the radial temperature gradients of a wafer and/or susceptor while ramping the temperature of the wafer and/or susceptor.